Since the manufacture of integrated circuits (IC's) emerged as an industry in the early 1960's, designers have been motivated to make their products faster and less expensive by making them smaller. Beginning with the development of a single planar transistor in 1959, there has been generally a straight line progression in the number of components per IC on the order of a four-fold increase every four years throughout the 1960's, 1970's, and 1980's. Attendant to the increased geometric density of electronic functions has been a steady decline in price of a given function and generally increased operating speed. Consequently, improvement of IC in both function and cost is largely dependent on the continued reduction of feature size.
Prior to the invention of planar transistors and integrated circuitry, switching elements were very expensive relative to means of conduction and dominated design theory. The advent of the IC has reversed relative costs to the extent that switching elements, transistors, and associated components are essentially free in an IC relative to costs of conducting paths (hereafter, interconnects or buses).
As the number of layers and transistors in a logic device such as a microprocessor or very high density random access memory continues to increase, there is a corresponding need to increase the various buses within the logic device. The buses can serve any of several purposes, for instance to provide power (V.sub.CC) or ground (V.sub.SS) interconnects, to provide a data interconnect or other interconnects, or to provide digit lines or other signals. As the number of transistors on the device increases with improved technology, the increased number of buses required will require a greater percentage of the available surface area.
One way semiconductor memory devices have been reduced in size is to use trench capacitors as storage nodes. Trench capacitors are square or round wells which are etched vertically into the wafer. By selectively doping the structure and laying down layers of conductor and dielectric (nonconductor) material, a capacitor node is fashioned which can store a charge.
Trenches have been used with success on chip designs to isolate transistors to prevent the unwanted tunneling of electrons between transistors. While they serve this function well, they occupy much needed chip surface area.